Interest in the mechanism of the peroxidative halogenation by myeloperoxidase (MPO) has arisen from recent reports. First, the physiological role of MPO in the killing of fungi, bacteria and viruses in the phagocytic process is dependent on peroxidative halogenation, particularly chloride ion because of its ubiquity; secondly, the detailed studies of M. Morrison (Miami Winter Symposium 8, 1974) with lactoperoxidase and L. P. Hager (ibid.) with chloroperoxidase have provided possible mechanisms with which to compare the activity of MPO. In contrast to the latter we wish to report that by using trapping experiments (Cl36), spectra measurements in the vapor phase, and by its odor, that free chlorine can be demonstrated in the MPO peroxidation of chloride ion. When coupled with hematoporphyrin as an acceptor, the formation of bile pigments can be shown as well as the oxidation of the heme of cytochrome C and the increased rate of choleglobin formation from the hemoglobin. Of particular significance the reaction with Cl provides evidence through its rate dependence on MPO concentration (10 to minus the 8th power - 10 to minus the 7th power M) that MPO dissociated at low concentration, that the decomposition of compound II is too slow to be compatible with the overall reaction rate, and that ternary complexes are involved in the halide peroxidation. Finally, the pH optimum for peroxidation of (I, Br, Cl) is 5-6, rather than 8 as reported for other peroxidations.